Device for use in exhaust aftertreatment system

ABSTRACT

A device for use in an exhaust aftertreatment system for an internal combustion engine is provided. The device comprises a particulate filter system having a particulate filter (DPF) for the collection of particles from the engine and a heating unit for the intermittent burning of soot at a temperature suitable for soot combustion. The particulate filter system is arranged to switch between an operating mode for the collection of soot and an operating mode for active regeneration. A bypass line extends between the inlet and outlet of the particulate filter system. A valve disposed in the bypass line allows opening or closing of an exhaust gas flow through the bypass line, in dependence on the operating mode of the particulate filter system, by a control member.

BACKGROUND AND SUMMARY

The present invention relates to a device for use in an exhaust aftertreatment system for an internal combustion engine, comprising a particulate filter system having a particulate filter (DPF) for the collection of particles from the engine and a heating unit for the intermittent burning of soot at a temperature suitable for soot combustion, the particulate filter system being arranged to switch between an operating mode for the collection of soot and an operating mode for active regeneration.

New emission requirements for diesel engines for heavy-duty vehicles such as trucks and buses mean that particulate filters which catch soot particles will begin to be used in the exhaust system. These filters may need to be cleaned of soot by a raising of the temperature so that the soot burns up. This generates exhaust gases with high temperature, which can be harmful to the environment in closed traffic environments, for example terminals.

Given forthcoming emission requirements, Nox-reducing catalysts will also be required in the exhaust system. If these are placed after the particulate filter, they may become damaged by the temperature produced by the soot combustion.

Various methods for avoiding harmfully high temperatures at the outlet from a DPF system are known. For example, the DPF system can be placed downstream of other exhaust aftertreatment systems. A drawback with this method is that soot can disturb the functioning in the NOx-reducing system. The problem of exhaust gases with high temperature which can be harmful to the environment remains with this method. Another method is to lead the exhaust gases past a following exhaust aftertreatment system via a bypass line. Here too, the problem of exhaust gases with high temperature which cap be harmful to the environment remains.

WO2006/126922 describes a system with the intention of keeping the temperature down after the DPF in soot regeneration. In this, the intake air of the engine is mixed with the exhaust gas from the DPF. A drawback with this solution is that it can affect the engine characteristics as a result of some of the available intake air being used for the aftertreatment system. A further drawback with this known solution is, in installation terms, that the air duct between the induction manifold and the exhaust pipe is long.

U.S. Pat. No. 4,665,690 describes an exhaust aftertreatment system with DPF, which comprises a valve with bypass line via the DPF system. An injector upstream of the DPF unit is used to supply a reaction agent to raise the temperature in the DPF unit. Here the bypass line is used to regulate a suitably constant flow through the DPF to prevent the DPF system from being damaged by the temperature in this becoming too high, which means that the valve should be continuously adjustable. This known device aims neither to protect any following aftertreatment system from high temperatures nor to protect the environment from exhaust gases with extremely high temperatures.

It is desirable to provide a simple and effective device which makes it possible to avoid the above-described problems.

A device according to an aspect of the invention is characterized by a bypass line extending between the inlet and outlet of the particulate filter system, a valve disposed in the bypass line, which allows opening or closing of an exhaust gas flow through the bypass line, and a control member for operating the valve in dependence on the operating mode of the particulate filter system. By virtue of this configuration of the device, a part-quantity of the exhaust gases can be led past the particulate filter and then remixed with the exhaust gas flow from the particulate filter for the duration of the soot combustion.

According to one advantageous illustrative embodiment of the invention, the valve is constituted by a so-called on/off valve, the bypass line being configured such that the part-flow through this line is sufficient to ensure, in the regeneration of the DPF system, that the temperature in the exhaust line downstream of this system is kept at a suitable level.

According to another advantageous illustrative embodiment of the invention, the valve is adjustable in steps or steplessly between different control positions.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in greater detail below with reference to illustrative embodiments shown in the appended drawings, wherein

FIG. 1 shows schematically a first illustrative embodiment of the device according to the invention,

FIG. 2 shows, in corresponding manner to FIG. 1, a second illustrative embodiment of the device according to the invention,

FIG. 3 is a diagram illustrating the exhaust temperature in soot combustion during a driving cycle, with non-adjustable valve, and

FIG. 4 is a diagram illustrating the exhaust temperature in soot combustion during a driving cycle, with adjustable valve.

DETAILED DESCRIPTION

FIG. 1 shows schematically an internal combustion engine 10, expediently a diesel engine, which is connected by an exhaust line 11 to a particulate filter system 12 comprising an oxidation catalyst (DOC) 13 and an actively regenerable particulate trap (DPF) 14. An injector 15 for hydrocarbons (HC), for example diesel fuel, is placed upstream of the DOC 13 and forms together with the latter a heating unit for intermittently burning soot at a temperature suitable for soot combustion. The particulate filter system is here arranged to switch between an operating mode for the collection of soot and an operating mode for active regeneration.

When it is time to regenerate the DPF, HC are injected into the exhaust gas stream by the injector 15 and are oxidized in the DOC. The temperature in the exhaust gas stream is here raised sufficiently to start a soot combustion in the DPF. Alternatively, the temperature increase in the exhaust gas stream in the regeneration can be achieved with the aid of a so-called burner.

The exhaust aftertreatment system of the engine can also comprise devices for Nox reduction, such as an EGR system for the recirculation of exhaust gas to the inlet side of the engine.

According to the invention, a bypass line 16 is disposed between the inlet and outlet of the particulate filter system 12, i.e. past the DOC 13 and the DPF 14. A valve 17 in the bypass line 16 allows opening or closing of the exhaust gas flow through the bypass line by a control member 18 for operating the valve 17 in dependence on the present operating mode of the particulate filter.

The control member 18 can be constituted, for example, by the electronic control unit of the engine, which also controls the switching between the two operating modes of the particulate filter system and activation of the injector 15. In its simplest form, the valve 17 can be of the on/off type.

With a valve 17 which only opens and closes, the valve is opened when the heating unit 15 of the particulate filter system is activated and is kept open until a time after the heating unit has been switched off. The time for closing of the valve is calculated such that the particulate filter has cooled. In this type of valve, it is advantageous if the bypass line contains a laminar flow resistor 19 in order to make approximately the same exhaust gas flow component pass through the bypass line irrespective of the operating point of the engine. The laminar flow resistor is dimensioned such that an adequate part-quantity is made to flow through the bypass line, somewhere within the range 20-70% depending on the maximum exhaust temperature of the engine and the desired maximum tailpipe temperature. If, for example, the exhaust temperature from the engine is 300 degrees C. and the temperature downstream of the particulate filter during regeneration is 700 degrees C., and if one half of the exhaust gases passes by the particulate filter system, exhaust gases with a temperature of 500 degrees C. are obtained following mixing-in of the part-flow from the bypass line 16.

FIG. 3 is a diagram which illustrates the exhaust temperature in soot combustion during a driving cycle with non-adjustable valve. Here, 40-50% of the exhaust gases pass through the bypass line 16.

The valve 17 can alternatively be adjustable in steps or continuously by the control member 18 according to FIG. 2, which schematically shows an internal combustion engine according to FIG. 1 with following actively regenerable particulate filter system 12 and, in addition, a SCR catalyst 20 using a reducing agent, for example urea, for Nox reduction. For this purpose, the reducing agent is mixed into the exhaust gas stream upstream of the SCR catalyst 20 from a tank by means of an injector 21.

Pressure sensors 22, 23 are placed upstream and downstream of the particulate filter system 12 to detect the fall in pressure over this system, which pressure fall can be used to determine when regeneration shall occur. Temperature sensors 24, 25 are also placed upstream and downstream of the particulate filter system 12 to detect the exhaust temperature firstly downstream of the engine and secondly downstream of the particulate filter system 12.

In the illustrative embodiment according to FIG. 2, an adjustable valve 17 is used, which, with the aid of preset values, allows adjustment to a given target value in dependence on the operating point and/or “closed loop” of the engine, with the aid of one of the temperature sensors 24, 25 after the mix-in point. The control unit 18 can also in this case be the same physical unit which regulates other parts of the aftertreatment system and the engine, otherwise there is, of course, a communication between these.

FIG. 4 shows examples of temperatures in soot combustion during a driving cycle with adjustable valve. In this case, 30-70% of the exhaust gases pass through the bypass line 16.

The shunting of exhaust gases via the bypass line 16 means that a certain quantity of exhaust gases will pass through the exhaust system without cleaning steps for particles. In order that this shall have a minimal effect upon the total emission picture, the soot combustion should not occur too often, but only a small percentage of the time. In order to minimize the quantity of soot, the following measures can be taken, one or more in combination:

-   -   The laminar flow limiter can consist of a so-called open filter         (removes about 50% of the soot particles).     -   When no following aftertreatment unit is present downstream of         the particulate filter system 12, the adjustment of the valve 17         can be dependent on the vehicle speed (rapid dilution of the         exhaust gases to low temperature at high vehicle speed).     -   Via the injection system for fuel to the cylinders, the engine         can be reset to produce less soot during the soot combustion.         The engine will then, however, produce a higher quantity of Nox,         but with a Nox aftertreatment unit which works at an optimal         temperature for this unit, the increased Nox quantity can then         be taken care of. For this purpose, the temperature which is         used as the target value after mixing-in should be the optimal         temperature of the NOx aftertreatment unit.

As a result of the device according to the invention, it becomes possible to have the DPF unit before the NOx-reducing unit, which keeps the NOx-reducing catalyst clean from soot and HC. Moreover, it becomes possible to exploit the fact that some soot is combusted with NO2, whereby the number of thermal soot combustions can be minimized. The installation is space-saving in comparison with an installation in which diluting air is taken from the induction side.

Moreover, the gas exchange of the engine is not adversely affected. In comparison with known solutions in which the NOx-reducing unit is bypassed, the invention provides freedom to choose which emission is wanted to be discharged (partially) uncleaned during the soot combustion, with the possibility of optimization via the control parameters of the engine. Moreover, the fuel consumption can be reduced in relation to known solutions in which NO2 combustion needs to be constant. The solution according to the invention also provides freedom to choose catalyst materials which may be better in respects other than regarding thermal stability.

Furthermore, fuel can be saved with the invention if the engine has such a combustion that no soot is formed (or less than emission requirements) in part of the load range, the shunting via the bypass line 16 reducing the fall in pressure of the exhaust system. If the bypass line and the valve 17 are configured so that the entire flow can pass through the bypass line, this can also be used as a safety measure to prevent damage to the DOC and DPF, such as overheating of the material and/or deactivation of any catalyst coating on the filter in the regeneration.

The invention should not be deemed to be limited to the above-described illustrative embodiments, but rather a number of further variants and modifications are conceivable within the scope of the following patent claims. 

1. A device for use in an exhaust aftertreatment system for an internal combustion engine, comprising a particulate filter system having a particulate filter (DPF) for the collection of particles from the engine and a heating unit for the intermittent burning of soot at a temperature suitable for soot combustion, the particulate filter system being arranged to switch between an operating mode for the collection of soot and an operating mode for active regeneration, a bypass line extending between the inlet and outlet of the particulate filter system, a valve disposed in the bypass line, which allows opening or closing of an exhaust gas flow through the bypass line, and a control member for operating the valve in dependence on the operating mode of the particulate filter system, so that the valve is open when the heating unit is activated and closed when the particulate filter has cooled down after such activation of the heating unit.
 2. The device as claimed in claim 1, wherein the valve is a so-called on/off valve, the bypass line being configured such that the part-flow through this line is sufficient so that, in the regeneration of the DPF system, the temperature in the exhaust line downstream of this system is kept at a suitable level.
 3. The device as claimed in claim 1, wherein the bypass line contains a laminar flow resistor.
 4. The device as claimed in claim 1, wherein the valve is adjustable between different control positions.
 5. The device as claimed in claim 4, wherein the valve is adjustable in steps.
 6. The device as claimed in claim 4, wherein the valve is steplessly adjustable.
 7. The device as claimed in claim 4, wherein a NOx-reducing catalyst placed downstream of the particulate filter.
 8. The device as claimed in claim 1, wherein the heating unit comprises a fuel injector.
 9. The device as claimed in claim 1, wherein the heating unit comprises a burner.
 10. The device as claimed in claim 9, wherein the heating unit additionally comprises an oxidation catalyst.
 11. The device as claimed in claim 2, wherein the bypass line contains a laminar flow resistor.
 12. The device as claimed in claim 2, wherein the valve is adjustable between different control positions.
 13. The device as claimed in claim 12, wherein the valve is adjustable in steps.
 14. The device as claimed in claim 12, wherein the valve is steplessly adjustable.
 15. The device as claimed in claim 12, wherein a NOx-reducing catalyst placed downstream of the particulate filter.
 16. The device as claimed in claim 2, wherein the heating unit comprises a fuel injector.
 17. The device as claimed in claim 2, wherein the heating unit comprises a burner.
 18. The device as claimed in claim 8, wherein the heating unit additionally comprises an oxidation catalyst. 